precision mediump float;
uniform sampler2D uTexture;
varying vec2 vTexCoord;

void main() {
    vec4 color = texture2D(uTexture, vTexCoord);
    float gray = 0.299 * color.r + 0.587 * color.g + 0.114 * color.b;
    gl_FragColor = vec4(gray, gray, gray, color.a);
}